A typical thermal transfer printer includes a supply of thermal ribbon which contains a backing layer and an ink (pigment, resin, wax, etc.) coating layer. The thermal ribbon is unwound from a supply reel and fed along a defined path such that the backing layer of the thermal ribbon comes into contact with a thermal print head where printing occurs. Opposite the thermal print head is a print roller. The gap between the thermal print head and the print roller defines a workstation where printing occurs. After passing by the thermal print head, the used thermal ribbon is collected on a take-up reel. Typically, the thermal ribbon, supply reel and take-up reel are located in a cassette which is detachably mounted to the thermal printer. Thus making it easy to insert a new cassette when the supply of thermal ribbon has been exhausted.
A combination of pressure and heat is necessary to produce a desired pattern on an article (paper, film, tape, etc.). A print or platen roller compresses the article against the thermal print head with the thermal ribbon captured therebetween such that the ink coating layer is in contact with the article. As the print roller rotates it causes the article to advance past the thermal print head. Because of compressive force and friction between the article and the thermal ribbon, the article drags the thermal ribbon along with it as it feeds past the thermal print head. Thus the article and the thermal ribbon move synchronously past the thermal print head with the thermal ribbon unwinding from the supply reel as necessary. As the article and the thermal ribbon feed past the thermal print head, a microcontroller selectively energizes individual heating elements of the thermal print head which cause the ink to liquefy. Because of the compressive force and greater attraction of the liquid ink to the article than to the thermal ribbon backing layer, the liquid ink transfers to the article where it cools and resolidifies. Depending on the timing and sequence of energizing the heating elements, any desired pattern (alphanumeric, barcode, postal indicia, etc.) can be transferred to the article. After printing, the used thermal ribbon is collected on the take-up reel.
In a thermal printer it is important to control the tension on the thermal ribbon between the thermal print head and the take-up reel. Once the ink transfers to the article and resolidifies, the thermal ribbon has a tendency to cling to the article. Therefore, it is necessary to peel the thermal ribbon from the just printed article. In extreme cases, too much tension will cause the thermal ribbon to deform, stretch or even break. On the other hand, too little tension also causes problems. During printing the thermal ribbon typically becomes wrinkled. Therefore, to ensure that the used thermal ribbon is neatly collected on the take-up reel adequate tension is necessary. Additionally, tension on the thermal ribbon between the thermal print head and the take-up reel also assists the print roller in feeding the article past the thermal print head. Thus, it is advantageous to control the tension on the thermal ribbon.
Additionally, some other factors that influence the tension requirements are: printing speed, type of ink being used, type of thermal ribbon being used, type of article being printed on, ambient environmental conditions and manufacturing tolerances.
Typical tension control systems seek to control the tension by applying a constant torque to the take-up reel. However, for a given torque applied to the-take-up reel, the resulting tension on the thermal ribbon, or other sheet material, is inversely proportional to the radius of the take-up reel as governed by the equation: EQU tension=torque/take-up reel radius.
In such systems, since the radius of the take-up reel increases as thermal ribbon is collected, the resulting tension on the thermal ribbon decreases. Therefore, a constant torque does not result in a constant tension.
Other tension control systems seek to achieve a constant tension by applying a variable torque to the take-up reel depending on the radius of the take-up reel. Therefore, the diameter of the take-up reel must be determined. Prior art mechanical approaches use spring loaded followers that are biased against the ribbon on the take-up reel where the position of the follower has a known relationship to the radius of the take-up reel. As a result, the changing radius of the take-up reel causes the follower to deflect to a new position from which the radius of the take-up reel can be ascertained. However, these mechanical approaches suffer from reliability and accuracy problems. Another problem is that they create drag on the take-up reel.
Still other approaches to determining the take-up reel radius utilize a known relationship between movement of the ribbon and either: (1) the angular rotation of the take-up reel over a given time period, or (2) the angular velocity of the take-up reel. Since both approaches are dependent on time, they must include apparatus to accurately measure elapsed time. Additionally, these approaches involve sophisticated measuring and feedback systems which add greatly to the overall cost of the product into which they are incorporated. Therefore, there is a need to control tension on a sheet material by techniques that are not dependent on time.